Gas-liquid contact trays are used for a variety of purposes in the field of chemical processing, including use in distillation columns to separate selected components from a multi-component stream. Gas-liquid contact trays are available in multiple varieties, including "cross-flow" configurations. In a cross-flow configuration, gas-liquid contact trays generally comprise a solid deck having a plurality of apertures. Liquid is deposited on and flows across at least a portion of the deck, and contacts vapor ascending through the apertures. The area in which the liquid contacts the vapor on the deck is designated the "active area." In using a gas-liquid contact tray, liquid is generally directed onto the deck through a channel, or "inlet downcomer."In a distillation column comprising a plurality of trays, the liquid may descend from a tray or other apparatus located higher in the column. Liquid exiting the deck is generally directed through a second channel, or "exit downcomer." In a distillation column comprising a plurality of trays, the liquid may descend to a tray or other apparatus located lower in the column. Decks may be divided into multiple sections, with each section being served by an inlet and exit downcomer.
"Weeping" and "liquid entrainment" are two potentially undesirable conditions which may occur in gas-liquid contact trays. Weeping occurs when liquid flowing across a deck flows into and through the apertures to lower levels of the processing apparatus. Liquid entrainment occurs when ascending vapor captures liquid flowing across a deck and carries the captured liquid to higher levels of the processing apparatus.
The reaction rates between the liquid and the vapor on a gas-liquid contact tray, as well as certain other hydraulic characteristics (such as liquid entrainment and weeping), are dependent upon the intimacy of contact and degree of intermixing between the liquid and the vapor on the deck of the gas-liquid contact tray. Two factors influencing the intimacy of contact and degree of intermixing are the size of the active area on a tray deck and the pattern of contact between the liquid and the vapor within the active area.
The size of the active area influences the intimacy of contact and degree of intermixing at least in part by influencing the possible contact time between the liquid and the vapor. Larger active areas generally improve the intermixing of a liquid and a vapor and generally increase the reaction rates between the fluid and the vapor. Longer active areas (measured parallel to the direction of liquid flow on the deck) clearly provide longer time periods for interaction between a gas and a liquid. Similarly, broader active areas (measured perpendicular to the direction of liquid flow on the deck) also allow longer time periods for interaction. The depth of the liquid flowing across the deck is closely regulated on many trays to prevent flooding of the processing apparatus. Though the ability to vary the liquid depth is substantially unavailable in such situations, the volume of liquid flowing over the deck may still be controlled by varying either the breadth of the area over which the fluid runs, or the velocity of the stream of fluid flowing through the area. When the breadth of the area is increased, the velocity of the stream may decrease, and thereby maintain the liquid on the tray deck for a longer period of time.
The pattern of the contact between the gas and liquid on a tray deck also influences the intimacy of contact and degree of intermixing. For example, uneven liquid flow patterns across a tray deck (including flow patterns wherein "eddies" are created along a deck's periphery) or, inconsistent vapor flow rates through the various apertures of a deck, may disadvantageously create inconsistent intermixing of liquid and gas. Further, the design of vapor/liquid contact mechanisms used on or with a tray deck, such as the size and configuration of the deck's apertures and the presence or absence of valve covers over those apertures, may also influence the pattern of contact between a gas and a liquid.
In the modem distillation industry two kinds of trays are often used: sieve trays and valve trays. Sieve trays are generally constructed with a large number of apertures. In sieve trays ascending vapor flows upward through the apertures to contact liquid flowing across the tray deck, and substantially no physical barrier exists between the apertures and liquid. The contact between vapor and liquid on the tray deck of a sieve tray is characterized by a shearing action between the ascending gas and cross-flowing liquid.
The diameter, or "thickness," of a vapor stream arising from a sieve tray aperture is naturally affected by the size of the aperture. Beneficially, a fixed volume of vapor ascending in a large number of thin streams of vapor will tend to have a larger contact area with the liquid through which the vapor passes than the same volume of vapor ascending in a smaller number of relatively thicker streams of vapor. Thus a larger number of thin streams ascending from smaller apertures may achieve better liquid/vapor intermixing and process efficiency than a smaller number of large streams ascending from larger apertures. The larger number of smaller apertures are also characterized as having a greater total "periphery length" (i.e. the sum of the perimeter lengths of the individual apertures) than the smaller number of larger apertures through which a commensurate volume of vapor passes.
However, the apertures of a sieve tray are typically constructed using an automatic punching machine operating at a very high speed, and the cost of aperture construction is dependent upon the size of aperture being constructed (smaller apertures being more costly). This higher cost associated with producing smaller apertures having a larger collective periphery length may be a limiting factor in selecting an aperture size for a sieve tray design.
Valve trays are constructed with valve covers mounted above the apertures. The valve covers may be "fixed" or "floating." Fixed valves are integrally attached to the tray deck. Floating valves are capable of a limited range of movement toward and away from the tray deck. Frequently floating valves move away from a tray deck in response to pressure exerted upon the floating valves by vapor ascending through the apertures above which the valves are mounted. In valve trays ascending vapor flows first upward through the apertures and then at least somewhat laterally due to the presence of the valve cover mounted above the aperture. More specifically, the vapor must flow through an "escape area" between the valve cover and the tray deck, and equal to the product of the perimeter length of the valve cover and the net rise between the upper surface of the tray deck and the lower surface of the valve cover.
Valve trays frequently utilize a smaller number of larger apertures than sieve trays, possibly in response to the relatively high cost of valve construction. Among valve trays, floating valve trays are generally more expensive to manufacture than fixed valve trays because each individual floating valve cover must be manually or mechanically installed, while fixed valve covers and the corresponding apertures may be constructed substantially simultaneously through mechanical processes such as punching. However, the speed of the mechanical punching processes utilized in the construction of even fixed valves are typically less than the speed of the mechanical punching processes utilized in the construction of sieve tray apertures and, therefore, the cost to make valve trays is generally significantly higher than the cost to make sieve trays. The utilization of relatively large net rises and aperture sizes on valve trays is one method of reducing the costs of valve tray construction.
A perceived benefit of valve trays over sieve trays is that vapor flowing exiting through a valve's escape area may flow at least somewhat laterally along the tray deck. The vapor may thus be thrust at least somewhat along the plane in which the liquid flows across the tray deck. The resulting pattern of vapor liquid contact may be superior to the shearing action found in sieve trays, because the depth of the liquid flowing over a tray deck may be very thin (with only froth located above the liquid) and ascending vapor in a sieve tray may have very little distance to rise before contact with the liquid is lost. However, despite the advantages in vapor/liquid intermixing realized through the use of valve trays, a need for improvement in valve cover design still remains. Specifically, the relatively large size and small number of apertures found in a valve tray may limit the total perimeter length of a valve tray's apertures and may produce thicker streams of vapor and smaller contact areas between the liquid and the vapor. Furthermore, in order to achieve a desired vapor/liquid handling capacity in a valve tray, a particular valve size and net rise may be indicated. A large net rise, in turn, may impede a valve cover's ability to beneficially transfer any lateral component to ascending vapor and force the vapor stream toward the tray deck.
To combat these negative effects, valve tray designers have recently sought to achieve better performance through the use of mini-valves having small net rises. The performance gained through the use of such mini-valves is accompanied by corresponding increase in manufacturing costs, and therefore a better answer in needed. Specifically, designs for fixed and floating valves are needed which can incorporate the benefits of thin vapor streams and can direct ascending vapor streams laterally along a tray deck (by limiting the required net rise while maintaining a desired escape area).